Path: utzoo!attcan!uunet!steinmetz!davidsen
From: davidsen@steinmetz.ge.com (William E. Davidsen Jr)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Leaving the hard disk on continuously
Message-ID: <11595@steinmetz.ge.com>
Date: 20 Jul 88 16:04:05 GMT
References: <12184@agate.BERKELEY.EDU> <1988Jul19.153220.854@mntgfx.mentor.com>
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Organization: General Electric CRD, Schenectady, NY
Lines: 46

In article <1988Jul19.153220.854@mntgfx.mentor.com> mikes@mntgfx.mentor.com (Mike Stanbro) writes:

  Here's a wild thought I had several months ago but never pursued it
  because it  seemed a bit risky.  There are two voltages to hard disks:
  +5V and +12V.   The +5V powers all of the electronic circuits, the +12V
  powers the spindle motor and actuator motor or coil.  If you shut the
  +12V off and leave the +5V on, the spindle  motor will shut down but
  the disk remains "intelligent" and will continue normally  after the
  +12V is reapplied and the platters spin back up to speed.
  
A bit risky indeed. Depending on the disk design it could be a small
problem of a large one. Consider the following disk types.

Head floats on air, lands at powerdown:

	Stopping the rotation would cause the heads to land every time.
Unless they were on the landing zone you could lose data. Most cheap
disks work like this, the only thing which keeps the head up is the flow
of air caused by disk rotation.

Head floats on air, lands on powerdown, parks on powerdown:

	These disks retract the head to the landing track before it
slows down enough to land the head. Cutting the motor power MAY park the
heads, but I wouldn't bet my disk on it.

Head normally retracted, floats on air:

	These disks retract the heads on powerdown, and only after the
disk is at speed do they move the head toward the platter to float on
the air cushion. This is usually found on top of the line drives. I
wouldn't bet that the motor power controls the heads, it might just be
the +5 for the electronics. In that case the heads would land, and the
head design wasn't intended to land... ever.

  You have an interesting idea, something like the opwering down of the
5-1/4" disks. Without having it built into the drive I think that there
is a good chance of disk damage, depending on the type.

Disclamer: my little list of disk types is incomplete and simplified,
don't (a) use it for more than examples, or (b) flame me for not being a
treatise on hardware.
-- 
	bill davidsen		(wedu@ge-crd.arpa)
  {uunet | philabs | seismo}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -me